The present invention relates to an improved speed detection apparatus having a rotation phase detector.
In general, in rotational drive apparatus such as rotating magnetic heads or the like, there are provided a speed detector (speed control circuit) and a phase detector that detect the speed of rotation and the phase of rotation of a rotating body, so that the speed of rotation of the rotating body such as a magnetic head drum or the like can be made a constant speed rotation of a required rotational phase, and each of these detector outputs is used in a drive motor having phase servo and speed servo. In this manner, a detection signal generator portion is formed by the combination of a FG (frequency generator) coil pattern, PU coil pattern and a rotor magnet.
The structure and principle of operation of a conventional speed detection apparatus having a rotation phase detector and having such a configuration will be described, with reference to FIG. 1 and later. FIG. 1 is a plan view of a rotor magnet 3 (one portion of a rotor) used in a conventional speed detection apparatus having a rotation phase detector, and 4 is a magnetized portion used for speed detection and formed in the direction of the circumference of the rotor magnet 3 for the greater portion (and hereinafter termed an "FG magnetized portion"), and 5 is a magnetized portion for detection of the phase and formed at portions other than those of the FG magnet 4, of those rotor magnets 3 (and hereinafter termed "PU magnetized portion"). The following is a description of the principle of operation in the conventional apparatus, with reference to FIG. 2.
FIG. 2(a) is a view describing the principle of operation, when the portion in the vicinity of the PU magnetized portion 5 of the rotor magnets 3, is extended in a line, and as shown in this figure, the PU magnetized portion 5 has a portion corresponding to two poles of the FG magnetized portion 4 divided equally into four, so as to form a pattern of a two-cycle portion. The FG magnetized portion 4 comprises 38 N or S poles. FIG. 2(b) shows a coil pattern 2 provided so as to oppose this rotor magnet 3, where 6 is an FG coil pattern (hereinafter termed an "FG pattern") for speed detection, and 7 is a PU coil pattern (hereinafter termed a "PU pattern") for phase detection.
In such a configuration, when the position relationship is as shown in FIG. 2(a), that is, when the FG pattern 6 is opposed to the FG magnetized portion 4, an FG signal is generated and is obtained across the terminal .alpha. and the terminal .beta., as shown in FIG. 2(c) . In addition, the PU signal is generated only when the PU pattern is opposed to the PU magnetized portion 5, as shown in FIG. 2(d), and is obtained across the terminals .gamma. and .delta.. In this case, the place where the pitch of the PU magnetized portion 5 and the pitch of the PU pattern 7 have a one-to-one correspondence is one per rotation. The place is provided to correspond to the reference position necessary for detection.
As has been described above, in a conventional speed detection apparatus having a rotation phase detector, the PU magnetized portion 5 does not contribute to the generation of FG signal when it is opposite the FG pattern 6. More specifically, when seen from the FG pattern 6, the status is the same as if the rotor magnet 3 had one non-magnetized portion, and an encoder error occurs when the center point of the FG pattern 6 is displaced from the rotational center of the rotor magnet 3. The cause of this (principle of occurrence) is described with reference to FIG. 3.
In FIG. 3, 6c is a circle of average radius R of the FG pattern 6, O is the center point of the FG pattern 6, and O' is a center of rotation of the rotor magnet 3. Here, the angular speed of rotation of the rotor magnet 3 is .omega. and the distance between both centers O and O' is r. Now, when r.noteq.0, that is, when the two centers O and O' are displaced each other, assuming that the FG magnetized portion 4 is around the entire circumference of the rotor magnet 3, speeds V.sub.1 and V.sub.2 shown in FIG. 8 are combined, and the average speed is V, according to the following equation. ##EQU1##
However, in reality, the FG magnetized portion 4 is in the status where there is a one pulse portion missing on the circumference of the rotor magnet 3 and so V.sub.1 and V.sub.2 do not combine. Here, the FG magnet can be thought of as having an encoder error in the status where there exists only a one-pulse portion. The other conditions can be the ideal status. More specifically,
(1) The FG pattern is of a type of integration for the entire circumference, wherein a true circle, and the line elements are equal to each other, with no surface deviation. PA1 (2) The FG magnet is divided equally with respect to the center of rotation, and the amount of magnetic flux generated by each pole is equal, and there is no surface deviation. PA1 (3) The speed of rotation of the rotor (magnet) is constant.
For these conditions, when the encoder error (fundamental speed of rotation component or the component per rotation per time) is made E.sub.e, then: ##EQU2##
In reality, since there are FG magnets in other portions, if the number of FG pulses is made n, then the influence of one pulse of the FG magnet becomes ##EQU3## Furthermore, if the average diameter of the FG pattern D=2R, then the FG pattern radial deviation d=2r and substituting this into the equation above gives the encoder error E.sub.e as follows. ##EQU4##
FIG. 4 shows the theoretical values (broken line) calculated by substituting the actual numerical value for the rotation speed detector into this equation, and the values (dots) actually measured. As can be seen from this figure, there is a positive correlation of significance level 1% between these two sets of values, and this can be understood as resulting from the encoder error as theoretically described above.
One attempt to solve this problem involves providing a portion where there is either no magnetism or a weak magnetism at a position that balances the PU magnetized portion 5, so as to reduce the encoder error but since this causes places where there is a large difference in the amount of magnetic field generated between adjacent pairs of poles of the FG magnetic portion 4, the PU noise (noise component included in the PU signals) becomes large and there is the disadvantage that this is a cause of malfunction.